Smooth muscle tissue is found in parts of the body where it conveys action without conscious intent. The majority of this type of muscle tissue is found in the Digestive and Urinary systems where it acts by propelling forward food, chyme, and feces in the former and urine in the latter. Other places smooth muscle can be found are within the uterus, where it helps facilitate birth, and the eye, where the pupillary sphincter controls pupil size.[2]

Agonist muscles and antagonist muscles refer to muscles that cause or inhibit a movement.

Agonist muscles cause a movement to occur through their own contraction. [4] For example, the triceps brachii contracts during the up phase of a push-up (elbow extension). During the down phase of a push-up, the same triceps brachii actively controls elbow flexion while relaxing. It is still the agonist, because while resisting gravity during relaxing, the triceps brachii continues to be the prime mover, or controller, of the joint action. (Agonists are also interchangeably referred to as "prime movers," since they are the muscles considered primarily responsible for generating a specific movement. This term typically describes skeletal muscles.[5])

Antagonist muscles oppose a specific movement. [6] This controls a motion, slows it down, and returns a limb to its initial position. Antagonism is not an intrinsic property; it is a role that a muscle plays depending on the motion. If a motion is reversed, agonist and antagonist muscles switch roles. Because a flexor muscle is always a flexor, in flexion it is the agonist, and in extension it is the antagonist. Conversely, an extensor muscle is the agonist in extension and the antagonist in flexion. Using the example above of the triceps brachii during a push-up, the elbow flexor muscles are the antagonists during both the up phase and down phase of the movement.[citation needed]

The antagonistic pair of biceps and triceps working to flex the elbow.

Antagonist and agonist muscles often occur in pairs, called antagonistic pairs. As one muscle contracts, the other relaxes. An example of an antagonisic pair is the biceps and triceps; to contract - the triceps relaxes while the biceps contracts to lift the arm. "Reverse motions" need antagonistic pairs located in opposite sides of a joint or bone, including abductor-adductor pairs and flexor-extensor pairs. These consist of an extensor muscle, which "opens" the joint (by increasing the angle between the two bones) and a flexor muscle, which does the opposite by decreasing the angle between two bones.

However muscles don't always work this way - sometimes agonists and antagonists contract at the same time to produce force, as per Lombard's paradox.

Not all muscles are paired in this way. An example of exception is the deltoid.

The biceps brachii flex the lower arm. The brachioradialis, in the forearm, and brachialis, located deep to the biceps in the upper arm, are both synergists that aid in this motion.

Synergist muscles perform, or help perform, the same set of joint motion as the agonists. Synergists muscles act on movable joints. Synergists are sometimes referred to as "neutralizers" because they help cancel out, or neutralize, extra motion from the agonists to make sure that the force generated works within the desired plane of motion.

Muscle fibers can only contract up to 40% of their fully stretched length. Thus the short fibers of pennate muscles are more suitable where power rather than range of contraction is required. This limitation in the range of contraction affects all muscles, and those that act over several joints may be unable to shorten sufficiently to produce the full range of movement at all of them simultaneously (active insufficiency, e.g., the fingers cannot be fully flexed when the wrist is also flexed). Likewise, the opposing muscles may be unable to stretch sufficiently to allow such movement to take place (passive insufficiency). For both these reasons, it is often essential to use other muscles, called fixators or synergists, in this type of action to fix certain of the joints so that others can be moved effectively, e.g., fixation of the wrist during full flexion of the fingers in clenching the fist. Synergists are muscles that facilitate the fixation action.

There is an important difference between a helping synergist muscle and a true synergist muscle. A true synergist muscle is one that only neutralizes an undesired joint action, whereas a helping synergist is one that neutralizes an undesired action but also assists with the desired action.[citation needed]

A muscle that fixes or holds a bone so that the agonist can carry out the intended movement is said to have a neutralising action. A good famous example of this are the hamstrings; the semitendinosus and semimembranosus muscles perform knee flexion and knee internal rotation whereas the biceps femoris carries out knee flexion and knee external rotation. For the knee to flex while not rotating in either direction, all three muscles contract to stabilize the knee while it moves in the desired way.

Composite or hybrid muscles have more than one set of fibers that perform the same function, and are usually supplied by different nerves for different set of fibers. For example, the tongue itself is a composite muscle made up of various components like longitudinal, transverse, horizontal muscles with different parts innervated having different nerve supply.

Smooth muscles are responsible for the contract-ability of hollow organs, such as blood vessels, the gastrointestinal tract, the bladder, or the uterus. It can develop isometric force per cross-sectional area that is equal to that of skeletal muscle. However, the speed of smooth muscle contraction is only a small fraction of that of skeletal muscle.[7]

Smooth muscle relaxation occurs either as a result of removal of the contractile stimulus or by the direct action of a substance that stimulates inhibition of the contractile mechanism (e.g., atrial natriuretic factor is a vasodilator).

The insertion and origin of a muscle are the two places where it is anchored, one at each end. The tissue of the attachment is called an enthesis.

The origin of a muscle is the bone, typically proximal, which has greater mass and is more stable during a contraction than a muscle's insertion. [8] For example, with the latissimus dorsi muscle, the origin site is the torso, and the insertion is the arm. When this muscle contracts, normally the arm moves due to having less mass than the torso. This is the case when grabbing objects lighter than the body, as in the typical use of a lat pull down machine. This can be reversed however, such as in a chin up where the torso moves up to meet the arm.

The insertion of a muscle is the structure that it attaches to and tends to be moved by the contraction of the muscle. [9] This may be a bone, a tendon or the subcutaneous dermal connective tissue. Insertions are usually connections of muscle via tendon to bone.[10] The insertion is a bone that tends to be distal, have less mass, and greater motion than the origin during a contraction.